Bioinspired Entangled Hydrogels with Evaporative and Radiative Cooling for Self-Adaptive Temperature Management

The development of passive temperature management in zero-energy consumption is of great significance for coping with the global energy and environmental crisis, and it has great potential to preserve food quality. Here, inspired by the guttation and evaporative cooling of plant leaves, an entangled porous hydrogel was synthesized by copolymerization of N-isopropylacrylamide (NIPAM) and butyl acrylate (BA) through a cross-linker-free strategy. The extremely low content of initiator led to sufficiently long chains and an entangled structure, maintaining the strong mechanical property of water-swollen hydrogels. The hydrogels were transparent at room temperature with visible transmittance higher than 84.90%, and they turned white at a high temperature with an average reflectance higher than 43.45% in the solar spectrum. The reversible transparency alteration brought a solar transmittance modulation of >63.92% and solar reflectance modulation of >43.99%. And the swollen hydrogels had a high emissivity (>93.05%) in the atmospheric windows. Due to the constricted network, free water was squeezed, and the water layer appeared on the hydrogel surface in a white mode, improving the water evaporation performance. Based on the integration of spectrum modulation, water evaporation, and radiative cooling, the hydrogels could achieve subambient temperatures under strong sunlight irradiance, showing a temperature difference of ∼6 °C in an outdoor experiment. Furthermore, the hydrogels preserved the pear quality by avoiding the sunburn damage and maintained a quality close to fresh samples. The cross-linker-free polymerization offered a feasible solution to improve the passive temperature management performance, and its enhanced mechanical properties also broaden application scenarios.